Edta Titration Background.docx

Testing the Hardness of Water Purpose: The purpose of this experiment is to determine the hardness of a water sample by measuring t he amount of calcium present. This analysis will utilize the method of titration. Introduction: All natural waters have salts dissolved in them. It is these salts that give water a unique taste. Sometimes the water can contain too much of these salts and can cause problems when the w ater is used for drinking or washing. These problems are caused mainly by the presence of calcium and magnesium ions in the water. You may have experienced this problem if you have taken a bath with water that contained too many calcium and magnesium salts. The problem is that you cannot get the soap to lather and form suds. When this happens, we say that the water is “har d.” In this experiment, you will analyze a sample of water to measure the amount of calcium (or th e water hardness) by performing titrations on water samples you provide from home or other sources. In the previous experiment, you learned how to perform a titration and use a buret so an in-depth explanation is not necessary. However, the reaction in this titration is not a neutralization reaction as in the acetic acid titration. In this case, an EDTA (ethylenediamine tetra-acetic acid) solution is the titrant, which will react or capture (also called chelate) the calcium ions in the water as shown in equation 1 below. Eqn. 1 (EDTA4-) (CaEDTA2-) The reaction between calcium ions and EDTA only occurs at a high pH; therefore, a solution th at

has a constant pH of 10 (called a buffer) will to be added to the water sample. Just as in the acid-base reactions, you must use an indicator to identify when the reaction is complete. In this experiment, Eriochrome black T, a metal ion indicator, will be used to visualize the endpointOH

+ Ca2+ A metal ion indicator is a compound whose color changes when it binds to a metal ion. For su ch an indicator to be useful in the titration of a metal with EDTA, the indicator must give up its metal ion to the EDTA. The indicator in its free form, i.e. not bound to any metal, is blue. A small amount of indicator is added to the solution containing the Ca2+ forming a wine red complex (Equation 2.) If we denote the indicator as In2-, we can write the reaction as: Ca2+ + In2- CaIn Eqn. 2 (blue) (wine red) CaIn + EDTA4- CaEDTA2- + In2- Eqn. 3 (wine red) (blue) In this experiment, the end point will be indicated when the original red solution turns to blue (Equation 3) indicating that the EDTA has reacted with all the calcium ions in the water sample. After the measurement, it will be necessary to calculate the amount of calcium ions present. Th e concentration units used to measure the water hardness is normally parts per million (ppm.) In this experiment you will measure calcium carbonate (CaCO3) in ppm. (One part per million is the same as 1 mL/L.) You will be able determine the hardness of your water sample by measuring the volume of your water sample and the volume (in mL) of the EDTA solution used to react with all of the calcium in the water sample. You will calculate the ppm of calcium carbonate in your water sample by using the following calculation

ppm CaCO3 = mL of EDTA used in titration X 1000 ppm mL of water sample Once the concentration in ppm is determined, it can be related to the hardness scale in Table 1 . Concentration (ppm) Hardness Rating < 61 Soft 61 – 120 Moderately hard 121-180 Hard > 180 Very hard